In electrophotographic imaging, a latent electrostatic image is formed on a photoconductive element. The image is then rendered visible by a development step in which the latent electrostatic image is contacted with a suitable developer mix.
One method for applying the developer mix is the magnetic brush process, as described in U.S. Pat. No. 3,795,618 to Kasper et al. In this method, developer material containing toner particles and magnetic carrier particles is carried by a magnet. The magnet's field causes the magnetic carrier particles to align in a brush-like configuration. When the "magnetic brush" is engaged with the electrostatic latent-image-bearing surface, the toner particles are drawn from the brush to the latent image by electrostatic attraction.
The role of the carrier particles is two-fold: (a) to transport the toner from the toner sump to the magnetic brush, and (b) to charge the toner by tribo-electrification. In an ideal electrophotographic system, the movement of the carrier particles is passive, i.e., under no circumstances should the carrier particles migrate from the magnetic brush onto the photoconductor. In a non-ideal or real life system, however, some carrier particles also leave the magnetic brush along with the toner particles and are deposited on the photoconductor. This phenomenon is known as "carrier pickup" or "developer pickup."
Several problems result from carrier pickup. First, because the toner laydown on the photoconductor governs the ultimate image quality, the presence of carrier particles among the toner particles in the developed image leads to image artifacts and generally poor image quality. Carrier pickup is particularly detrimental in color applications, because the carrier particles will appear as black specks in otherwise homogeneous color images. In addition, the hard carrier particles become partially entrapped on the relatively soft photoconductor surface, causing permanent local damage to the photoconductor. A damaged photoconductor further enhances the generation of image artifacts.
Generally, the carrier particles which migrate from the magnetic brush onto the photoconductor are much smaller than the mean particle size of the carrier particles. Because of their small size, these particles can be charged to the same sign and extent as toner particles, leading to carrier pickup. Such small particles are produced during the conventional manufacturing process. Therefore, it is very important to eliminate, or significantly reduce, the generation of these small particles, known as "carrier fines", during the carrier manufacturing process.
In the conventional carrier manufacturing process, the constituent metal oxide and other metal salt particles are mixed in a predetermined ratio. This base material is then mixed with a solution of guar gum in water. Guar gum is a natural product which has been widely used in industry because it is inexpensive, non-toxic, soluble, and generally available. It also undergoes nearly complete combustion in the subsequent firing stage, leaving little residue in the magnetic ferrite carrier particles.
The mixture of the constituent metal salts and the guar gum solution is ball milled into a liquid slurry, which is spray dried to form the unreacted nonmagnetic, dried green beads. Spray drying is the most commonly used technique to manufacture green beads. The technique is described in K. Masters, Spray Drying Handbook, George Godwin Limited, London, 1979, which is hereby incorporated by reference.
The green beads are subsequently cured or fired at high temperatures, generally between 900.degree. C. to 1500.degree. C. During the firing process, the individual metal salt particles within the individual green beads react to produce the proper crystallographic phase. The magnetic properties of the carrier particles are dictated by the properties of the desired crystallographic phase.
In addition to the problem of carrier pickup, the magnetic properties of the carrier particles can be adversely affected by the generation of fines during the spray drying process. During the firing process, the individual unreacted constituent metal salts bound in the nonmagnetic green bead react to form a magnetic carrier particle. The magnetic character of the carrier particle is controlled by the chemical stoichiometry of the constituting oxides. For optimum carrier performance, it is important that the chemical composition of the green beads be maintained throughout the spray drying process. The disintegration of green beads can result in chemically heterogeneous green particles, which will lead to less than optimum chemical reactions during the firing process, and inferior magnetic performance of the final product.
In the process of manufacturing magnetic carrier particles, it is possible to control firing temperatures and times to avoid over-sintering particles and producing firing-process-induced fines. The generation of fines during atomization for spray-drying can also be eliminated by proper selection of operating conditions, such as the rotational velocity of the disk, fluid pressure and viscosity, and inlet/outlet temperatures. Despite these precautionary efforts, however, extensive fine generation occurs during the manufacturing process.